There are two known prior art methods for measuring the on-chip temperature of a FET based integrated circuit. The first is to use thermal diodes, which may be on-chip or near the chip being monitored. This method requires a well characterized and stable diode. For measuring the on-chip temperature of a FET, a gate schottky diode, a gate p-n diode, a source to body diode, or a drain to body diode may be used. These on-chip thermal diodes work well when the current-voltage (IV) characteristic has a known dependence on temperature. Most technologies based on silicon rely on thermal diodes to determine the on-chip temperature. However, generally only a small number and sometimes only one thermal diode is placed on-chip so only an average chip temperature can be measured. This may not be sufficient when many circuits need to be monitored. Even if more thermal diodes are placed on-chip, the proximity of the thermal diodes to the location on a circuit to be measured is generally limited to distances much greater than a micron, which lowers the accuracy of the temperature measurement.
Thermal diodes can provide reasonably accurate continuous real-time data; however, a key disadvantage with this method is that many compound semiconductors, such as GaN, SiC, and GaAs, are not mature enough to have well characterized and stable thermal diodes.
Another method is to use an infrared (IR) camera. This method involves using a rather bulky camera that is usually several thousand times the volume of the actual integrated circuit to produce a 2D temperature profile of the integrated circuit surface. Because this method requires a camera, it is suitable for monitoring the temperature of developmental and engineering prototypes, but not for monitoring the temperature of integrated circuits in use in commerce. Another disadvantage is that the IR radiation used by the camera limits the spatial resolution to approximately 10 microns; however, the region of interest may have critical dimensions less than one micron. Furthermore, this method only provides the average integrated circuit surface temperature within the spatial resolution of the IR camera.
Accurate monitoring of the temperature of FETs during operation is especially important when they are used in high power applications such as in electric vehicles. In the prior art substantial amounts of cooling equipment are used to maintain the temperature of the FETs below their theoretic capability. By monitoring the temperature of the FETs during operation, the FETs can be safely operated at a higher temperature thereby reducing the amount of cooling equipment and saving cost and weight.
What are needed are test structures and methods for accurately monitoring the channel temperature of FETs. Also needed are test structures and methods for monitoring the channel temperature of FETs built with compound semiconductors, such as GaN, SiC, and GaAs. The embodiments of the present disclosure answer these and other needs.